1. Field of the Invention
The present invention relates to an aqueous emulsion or dispersion type photosensitive resin composition which is highly suitable for production of printing plates, especially screen printing plates.
2. Description of the Prior Art
Various photosensitive resin compositions have been used as printing plate materials, photoresists for photo-etching and vehicles for paints and printing inks. Especially, photosensitive resin compositions as materials for printing plates are required to satisfy not only basic requirements such as high sensitivity and high resolving power but also printing resistance sufficient to resist stresses such as friction or compression under application of an oily or aqueous ink, good adhesion to a substrate and good storage stability.
Known photosensitive resin compositions to be applied to the above mentioned uses are roughly divided into the solvent development type, the aqueous alkali development type and the water development type. Most of these known photosensitive resin compositions are homogeneous photosensitive compositions, and aqueous emulsions are used in some of the water development type compositions. For example, a photosensitive resin composition comprising an aqueous emulsion of a polymer such as partially saponified polyvinyl acetate and a photo-crosslinking agent such as a dichromate or a diazo resin is used particularly for a screen printing plate.
However, such conventional aqueous emulsion type photosensitive resin compositions are also accompanied by problems in that (a) the solvent resistance of a basic polymer constituting an aqueous emulsion is insufficient, (b) since the amount of partially saponified polyvinyl acetate should be increased so as to prevent reduction of the solvent resistance of the photosensitive resin composition, the water resistance is reduced and (c) a relatively large amount of a surface active agent is used as an emulsifier for the polymer emulsion. For these reasons, cured or hardened films having excellent water resistance, solvent resistance and printing resistance cannot be prepared from the conventional aqueous emulsion type photosensitive resin compositions.
As a water-soluble, high-sensitivity photosensitive polymer having a photo-crosslinking property, there is known a polymer formed by introducing a styrylpyridinium group into a polymer chain of partially saponified polyvinyl acetate (see Japanese patent application Laid-Open Specification Nos. 23163/80 and 62905/80), and there is also known an aqueous emulsion type photosensitive resin composition formed by adding this photo-crosslinkable polymer, instead of the photo-crosslinking agent such as a dichromate or a diazo resin, to an emulsion of a polymer such as polyvinyl acetate (see Japanese patent application Laid-Open Specification No. 62446/80). A composition of this type has excellent sensitivity and storage stability, but from the results of researches we have conducted, it has been found that since an emulsified polymer is used as the main base polymer, several problems described below arise.
In the case where a hardened product obtained by using a polyvinyl acetate emulsion as a typical polymer emulsion is used for fabrication of a printing plate, this plate has insufficient oil resistance and is readily corroded by an organic solvent contained in an ink. Furthermore, the plate has insufficient water resistance, and corrosion thereof is promoted in a state where water is incorporated in the organic solvent. Moreover, at the step of mixing an emulsion such as an acrylic acid ester resin emulsion, an ethylene-acrylic acid ester copolymer resin emulsion, an SBR latex, a silicone resin emulsion, a vinyl chloride resin emulsion or a vinylidene chloride emulsion with an aqueous solution of saponified polyvinyl acetate to which a styrylpyridinium group has been added, such troubles as increase of the viscosity, gelation, dispersion fracture of the emulsion and formation of coarse particles arise. In the case of each of the foregoing emulsions, since a relatively large amount of a surface active agent is used, the solvent resistance and water resistance of the hardened product obtained are degraded by the presence of the surface active agent.